Anodizing of aluminum is most commonly performed in sulfuric-acid based solutions, for example, using processes defined as “Type II” by U.S. MIL-A-8625 specifications. The resultant anodic oxide coatings generally provide good wear and corrosion resistance to the aluminum substrate. The anodic oxides are also conducive to taking on dyes for coloring. On some aluminum alloys, and within certain process constraints, the resulting anodic oxides from a type II anodizing process may be clear and substantially colorless, giving a bright metallic appearance that is desirable in many products. Thus, type II anodizing is widely used in various industries.
It has been found, however, that using a type II anodizing process on certain types of aluminum alloys can result in anodic oxides that are slightly discolored due to presence of certain types of alloying elements within the aluminum alloys. This slight discoloration may be acceptable for some products where precise coloring is not required. However, in consumer products where finish coloring and color matching of product lines is of utmost importance, such discoloration can be highly undesirable. What is needed therefore are methods of anodizing certain types of aluminum alloys such that discoloration due to alloy elements is minimized or negated.